Perera H, Williamson J F, Li Z, Mishra V, Meigooni A S
Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO 63110.
Int J Radiat Oncol Biol Phys. 1994 Mar 1;28(4):953-70. doi: 10.1016/0360-3016(94)90116-3.
Ytterbium-169 (169Yb) is a promising new isotope for brachytherapy with a half life of 32 days and an average photon energy of 93 KeV. It has an Ir-192-equivalent dose distribution in water but a much smaller half-value layer in lead (0.2 mm), affording improved radiation protection and customized shielding of dose-limiting anatomic structures. The goals of this study are to: (a) experimentally validate Monte Carlo photon transport dose-rate calculations for this energy range, (b) to develop a secondary air-kerma strength standard for 169Yb, and (c) to present essential treatment planning data including the transverse-axis dose-rate distribution and dose correction factors for a number of local shielding materials.
Several interstitial 169Yb sources (type 6) and an experimental high dose-rate source were made available for this study. Monte-Carlo photon-transport (MCPT) simulations, based upon validated geometric models of source structure, were used to calculate dose rates in water. To verify MCPT predictions, the transverse-axis dose distribution in homogeneous water medium was measured using a silicon-diode detector. For use in designing shielded applicators, heterogeneity correction factors (HCF) arising from small cylindrical heterogeneities of lead, aluminum, titanium, steel and air were measured in a water medium. Finally, to provide a sound experimental basis for comparing experimental and theoretical dose-rate distributions, the air-kerma strength of the sources was measured using a calibrated ion chamber. To eliminate the influence of measurement artifacts on the comparison of theory and measurement, simulated detector readings were compared directly to measured diode readings. The final data are presented in the format endorsed by the Interstitial Collaborative Working Group.
The in-air calibration revealed that the air-kerma strength per unit activity (mCi), as quoted by the vendor, varied from 1.30 to 1.57 cGy.cm2/mCi.h depending on seed design. The maximum difference between measured and MCPT-simulated absolute diode readings on the transverse axis was less than 2%, indicating that MCPT accurately predicts dose rate in medium for brachytherapy sources in this energy range. Comparison of measured and simulated HCFs for each of the 16 different cylindrical heterogeneities demonstrated 1-3% agreement. The HCFs vary by as much as 200% with respect to distance and by as much as 48% as a function of disk diameter, showing that HCF is strongly dependent on heterogeneity location and lateral dimensions as well as thickness. The dose-rate constant for water medium was found to be 1.225 cGy/h per kerma unit air-strength and 1.962 cGy/h per unit mCi as measured by the vendor.
Monte Carlo simulation is an accurate and powerful tool for dosimetric characterization of brachytherapy sources in this energy range. Thin lead foils produce shielding factors comparable to standard shielded applicators for 137Cs. Meaningful theoretical absolute dose calculations in brachytherapy require accurately implemented air-kerma strength standards.
镱 - 169（169Yb）是一种很有前景的近距离放射治疗新同位素，半衰期为32天，平均光子能量为93千电子伏特。它在水中具有与铱 - 192等效的剂量分布，但在铅中的半价层要小得多（0.2毫米），能提供更好的辐射防护以及对剂量限制解剖结构的定制屏蔽。本研究的目标是：（a）通过实验验证该能量范围内的蒙特卡罗光子输运剂量率计算；（b）为169Yb制定二级空气比释动能强度标准；（c）提供重要的治疗计划数据，包括一些局部屏蔽材料的横轴剂量率分布和剂量校正因子。
本研究使用了几个间质169Yb源（6型）和一个实验性高剂量率源。基于经过验证的源结构几何模型的蒙特卡罗光子输运（MCPT）模拟用于计算水中的剂量率。为了验证MCPT预测结果，使用硅二极管探测器测量均匀水介质中的横轴剂量分布。为了用于设计屏蔽施源器，在水介质中测量了由铅、铝、钛、钢和空气的小圆柱形不均匀性引起的不均匀性校正因子（HCF）。最后，为了为比较实验和理论剂量率分布提供可靠的实验基础，使用校准的电离室测量源的空气比释动能强度。为了消除测量伪影对理论与测量比较的影响，将模拟探测器读数直接与测量的二极管读数进行比较。最终数据以间质协作工作组认可的格式呈现。
空气中的校准表明，根据种子设计，供应商提供的每单位活度（毫居里）的空气比释动能强度在1.30至1.57 cGy·cm²/mCi·h之间变化。横轴上测量的和MCPT模拟的绝对二极管读数之间的最大差异小于2%，表明MCPT能准确预测该能量范围内近距离放射治疗源在介质中的剂量率。对16种不同圆柱形不均匀性各自测量的和模拟的HCF进行比较，显示二者吻合度为1 - 3%。HCF随距离变化高达200%，随盘直径变化高达48%，表明HCF强烈依赖于不均匀性位置、横向尺寸以及厚度。通过供应商测量，发现水介质的剂量率常数为每比释动能单位空气强度1.225 cGy/h，每单位毫居里1.962 cGy/h。
蒙特卡罗模拟是表征该能量范围内近距离放射治疗源剂量学特征的准确且强大的工具。薄铅箔产生的屏蔽因子与137Cs的标准屏蔽施源器相当。近距离放射治疗中有意义的理论绝对剂量计算需要准确实施空气比释动能强度标准。
